What Causes Sedimentation in SC Pesticides?

Sedimentation occurs in SC pesticides because the active ingredient is present as solid particles suspended in a liquid phase rather than completely dissolved.

Gravity naturally drives those particles downward over time. However, the rate and structure of settling depend on much more than gravity alone.

Particle size, density difference, rheology, dispersant performance, active ingredient loading, temperature and particle interactions all influence whether an SC develops a soft, redispersible sediment or a dense hard cake.

Visible sediment therefore does not automatically mean an SC pesticide has failed.

The more useful questions are:

• How quickly is the product settling?
• Does the sediment remain soft?
• Can the formulation be adequately redispersed?
• Has crystal growth or agglomeration occurred?
• Does the product still meet its physical specification?

Sedimentation is one part of the wider topic of suspension stability in SC pesticide formulations, but understanding its specific causes helps explain why apparently similar SC products can behave very differently during storage.

Why Do SC Pesticides Naturally Tend to Settle?

SC stands for Suspension Concentrate.

In this formulation type, fine active ingredient particles are distributed throughout a continuous liquid phase.

The formulation may also contain:

• dispersants;
• wetting agents;
• rheology modifiers;
• antifoaming agents;
• preservatives;
• other stabilizing components.

Because the active ingredient particles are physically separate from the liquid, gravity creates a natural tendency for them to move downward.

Formulators cannot simply eliminate gravity.

Instead, they design the suspension so that settling occurs slowly enough and remains sufficiently reversible for the product to maintain acceptable storage and use characteristics.

A technically successful SC therefore does not necessarily mean:

No particle ever moves downward.

It means the complete system controls settling, particle interaction and redispersion well enough for the formulation to remain usable and within specification.

Which Factors Control Sedimentation in an SC Formulation?

SC sedimentation is not controlled by one parameter.

Several variables interact at the same time.

No single row in this table should be interpreted independently.

A formulation can have fine particles and still settle if its rheological or dispersant system is weak.

How Does Particle Size Affect Sedimentation?

Particle size is one of the most obvious factors influencing settling.

Under otherwise comparable conditions, larger particles generally have a stronger tendency to move downward under gravity than smaller particles.

Reducing particle size can therefore help manage rapid sedimentation.

But that does not mean that continuously reducing particle size solves the problem.

Very fine particles have higher total surface area and stronger particle-surface interactions. If those surfaces are not adequately stabilized, the particles can flocculate or agglomerate into larger effective structures.

This is why SC quality depends on a controlled particle-size distribution, not simply the smallest possible Dv50.

POMAIS explains this relationship in more detail in How Particle Size Affects SC Pesticide Performance.

How Does Density Difference Affect SC Settling?

The difference between the density of the solid active ingredient and the density of the surrounding liquid also influences sedimentation.

When suspended solid particles are substantially denser than the continuous phase, gravity creates a stronger tendency for them to move downward.

Basic settling theory often uses Stokes-type relationships to illustrate why factors such as:

• particle diameter;
• density difference;
• liquid viscosity

affect settling velocity.

However, a commercial SC is not an ideal dilute suspension.

Many SC formulations are concentrated, structured and non-Newtonian. Particle interactions, flocculation and rheology can strongly modify actual settling behavior.

For this reason, a simple Stokes-law calculation cannot reliably describe the full sedimentation behavior of a commercial pesticide SC.

It is useful for understanding direction, but not sufficient for predicting real storage stability.

Why Are Viscosity and Rheology Important?

Viscosity matters, but viscosity alone does not define suspension stability.

This distinction is important.

A low-viscosity continuous phase may provide little resistance to particle movement, potentially allowing more rapid settling.

It may seem logical to solve this by simply increasing viscosity.

But an excessively viscous SC can create other problems:

• poor pourability;
• difficult pumping;
• difficult mixing;
• slower container emptying;
• poor redispersion;
• treatment-equipment problems.

A better SC uses appropriate rheology.

The formulation may be structured enough while standing still to slow particle movement, but become easier to flow when subjected to shear during shaking, pumping or pouring.

This kind of behavior is more useful than simply making the product “thick.”

The technical objective is therefore:

sufficient structure at rest + acceptable flow under use conditions

rather than maximum viscosity.

How Do Dispersants Affect Sedimentation?

Dispersants help stabilize the surfaces of suspended particles.

Without adequate stabilization, particles can attract each other and form larger structures.

This can lead to:

• flocculation;
• agglomeration;
• particle growth;
• faster settling;
• compact sediment.

However, flocculation should not automatically be described as completely undesirable.

Some controlled particle-network structures may help prevent dense compact packing at the bottom of a container.

The real formulation problem is uncontrolled particle association that leads to unstable rheology, rapid phase separation or poor redispersibility.

A stable SC therefore requires balance.

The dispersant system must work together with:

• particle size;
• solids loading;
• continuous phase;
• rheology modifiers;
• active ingredient surface chemistry.

This is another reason why comparing only particle size or only viscosity gives an incomplete picture.

How Does Active Ingredient Loading Change Settling Behavior?

SC formulations can contain a high concentration of suspended solids.

As the amount of solid material increases, particle-particle interactions become more important.

The relationship between concentration and sedimentation is not always linear.

At relatively high particle concentrations, individual particles may interfere with each other’s movement.

This phenomenon is often described as hindered settling.

Instead of each particle falling independently through the liquid, the concentrated particle population collectively slows movement.

That does not mean high-solids SC formulations are automatically stable.

Higher loading can also create challenges involving:

• viscosity;
• milling;
• dispersion;
• particle interaction;
• pumping;
• redispersion.

The important point is that solids concentration changes the physical system itself.

A high-loading SC cannot be understood simply by applying the behavior of a dilute suspension.

How Do Crystal Growth and Agglomeration Affect Sedimentation?

Particle size at the time of manufacture may not be the same as particle size after long-term storage.

Two processes are particularly important.

Agglomeration

Individual particles can associate and form larger structures.

Even if the original particles are fine, agglomeration increases their effective size.

This can alter:

• settling behavior;
• wet-sieve residue;
• redispersibility;
• viscosity.

Crystal Growth

Some active ingredients have limited solubility in the liquid phase.

Under certain storage conditions, material can gradually move from smaller particles toward larger crystals.

This can increase the coarse fraction of the particle-size distribution.

As crystals grow, the formulation may become more vulnerable to:

• faster settling;
• hard sediment;
• wet-sieve problems;
• poor suspension uniformity.

This is why a formulation that looks stable immediately after milling may still require storage testing.

POMAIS covers the broader connection between physical change and product aging in How Pesticide Formulation Affects Shelf Life.

How Does Temperature Affect Sedimentation?

Temperature changes several properties at the same time.

High Temperature

Higher temperature may reduce viscosity in some systems.

If the continuous phase becomes less resistant to particle movement, settling may become faster.

Heat may also influence:

• active ingredient solubility;
• dispersant behavior;
• crystal growth;
• chemical stability;
• rheological structure.

Low Temperature

Low temperatures can produce different effects.

Depending on the formulation, they may cause:

• viscosity increase;
• crystallization;
• structural changes;
• altered dispersion behavior.

Therefore, it is too simplistic to say:

High temperature causes settling and low temperature prevents it.

The effect depends on the complete formulation.

Temperature stability must be evaluated experimentally under the storage conditions relevant to the product specification.

Is Sedimentation Always a Sign of Poor SC Quality?

No.

Sedimentation alone is not sufficient to diagnose formulation failure.

The structure of the sediment matters much more.

Soft Settling

Soft settling occurs when particles accumulate at the bottom but remain relatively loosely packed.

A soft sediment may be capable of returning to a reasonably uniform suspension after appropriate agitation.

This can be compatible with acceptable SC behavior if the product continues to meet its specification.

Hard Caking

Hard caking is more serious.

Particles form a dense, compact sediment that becomes difficult or impossible to redisperse.

This can create:

• uneven active ingredient concentration;
• incomplete container emptying;
• inconsistent dosing;
• poor dilution;
• handling problems.

The distinction is therefore:

visible sediment ≠ automatically bad

but:

poorly redispersible hard sediment = significant stability concern

Why Can a Fine-Particle SC Still Sediment?

Because particle size is only one part of the system.

An SC can have a fine initial particle-size distribution and still develop sediment if:

• the continuous phase has insufficient rheological structure;
• dispersant performance is poor;
• particles flocculate;
• the density difference remains high;
• crystal growth occurs during storage;
• the product is stored for long periods;
• temperature changes alter the formulation.

This is why:

fine particle size does not guarantee low sedimentation

and:

low sedimentation does not automatically prove high SC quality

For example, an extremely high-viscosity product may show little visible settling but still have unacceptable pouring, mixing or dilution behavior.

Formulation quality must be assessed as a system.

Sedimentation vs Suspensibility: What Is the Difference?

These terms are related but not interchangeable.

Sedimentation describes a physical process.

Suspensibility is a defined formulation-performance property measured under specified test conditions.

A product can therefore show some storage sedimentation in its original container while still producing acceptable suspensibility after correct redispersion and dilution.

This distinction is essential when interpreting SC performance.

Sedimentation in the Container vs After Dilution

Sedimentation can occur in two different environments.

They should not be treated as the same problem.

Sedimentation During Storage

In the original concentrated product, the main concerns include:

• long-term physical stability;
• sediment structure;
• caking;
• redispersibility;
• pourability;
• container residue.

The formulation is highly concentrated and its rheology may be very different from a diluted spray mixture.

Settling After Dilution

After an SC is added to the spray tank, particle concentration changes dramatically.

The key concerns become:

• suspensibility;
• agitation;
• water dispersibility;
• spray mixture uniformity;
• coarse material;
• application consistency.

A product can behave differently in these two environments.

This is why bottle sedimentation should not be used as a direct substitute for suspensibility testing.

How Is Sedimentation-Related SC Stability Evaluated?

No single test provides a complete answer.

Several formulation tests help describe different parts of sedimentation-related performance.

Suspensibility

Suspensibility evaluates how well the active ingredient or insoluble material remains suspended after the product is diluted under defined conditions.

It is especially relevant to spray-tank performance.

Pourability

Pourability evaluates whether a suspension concentrate can be removed from its container after standing under specified conditions.

It can identify problems related to:

• excessive sediment;
• container residue;
• difficult product flow.

Wet Sieve

Wet-sieve testing identifies excessive coarse material.

Large particles, crystals or agglomerates may indicate physical instability and can also create application-equipment concerns.

Storage Stability

Accelerated and low-temperature storage testing can reveal changes such as:

• sedimentation;
• viscosity drift;
• crystal growth;
• phase behavior;
• redispersion problems.

Redispersibility

Where relevant to the product specification, redispersibility helps determine whether settled material can be returned to a sufficiently uniform suspension.

Together, these tests provide a much more useful picture than simply observing whether a bottle contains sediment.

What Should Be Evaluated Alongside Sedimentation?

Sedimentation should be interpreted together with other SC parameters.

A commercial SC formulation should therefore be evaluated through a combination of physical parameters.

For example, a commercial low-settling SC formulation can be designed around particle-size control, rheology, suspensibility, redispersion and storage stability together rather than relying on one sedimentation observation.

Can Rheology Prevent Sedimentation?

Rheology can help control sedimentation, but it cannot guarantee stability by itself.

A well-designed rheological system can slow particle movement while keeping the formulation pourable and pumpable.

However, if particles are poorly dispersed, excessively coarse or undergoing crystal growth, rheology alone may not solve the underlying instability.

The better formulation principle is:

particle size + dispersant system + rheology + solids loading + storage stability

must work together.

This is more accurate than saying:

Add more thickener to prevent settling.

Increasing viscosity without understanding the complete system can simply replace one quality problem with another.

Frequently Asked Questions

Why do SC pesticides settle during storage?

SC pesticides contain solid active ingredient particles suspended in liquid. Gravity causes a natural settling tendency, while particle size, density difference, rheology, dispersant performance, concentration and storage conditions determine how fast and how densely the particles settle.

Does sedimentation mean an SC pesticide has gone bad?

No. Some soft, reversible sediment can occur in suspension systems. The more important concern is whether the sediment becomes hard, difficult to redisperse or causes the product to fail its specification.

Can a fine-particle SC still sediment?

Yes. Fine particles can still settle if rheology is weak, particles flocculate, density difference is high, crystal growth occurs or the formulation is stored for a long period.

Does higher viscosity always reduce sedimentation?

Not necessarily. Higher viscosity can slow particle movement, but excessive viscosity may create pouring, pumping, mixing and redispersion problems. Appropriate rheology is more important than maximum viscosity.

What is the difference between sedimentation and suspensibility?

Sedimentation is the physical movement of particles downward. Suspensibility is a defined test property describing how well the diluted formulation remains suspended under specified conditions.

Can a settled SC pesticide be redispersed?

Some can. Soft sediment may return to suspension with appropriate agitation, while hard caking can be difficult or impossible to redisperse. Product acceptability should be judged against the applicable specification rather than visual appearance alone.

What Really Controls Sedimentation in an SC Pesticide?

Sedimentation is a natural consequence of keeping solid active ingredient particles suspended in a liquid formulation.

The challenge is not simply to eliminate all visible settling.

A technically sound SC needs to control:

• particle-size distribution;
• density effects;
• particle interactions;
• dispersant performance;
• rheology;
• viscosity;
• crystal growth;
• storage behavior.

The most important distinction is between manageable, reversible settling and irreversible hard caking.

A product with some soft sediment may still perform correctly after redispersion, while a product with little visible sediment can still have other formulation problems.

For that reason, sedimentation should always be evaluated together with suspensibility, redispersibility, pourability, wet-sieve results and storage stability.

This system-level approach provides a much more reliable picture of SC formulation quality than simply asking whether sediment is visible.

For commercial crop-protection programs, POMAIS provides a range of SC pesticide formulations developed for different active ingredients, crops and registered market requirements.